This invention relates to a novel method for the production of presensitized lithographic printing plates and to the novel lithographic printing plates produced thereby. More particularly, this invention relates to a novel method of producing aluminum based photosensitive lithographic printing plates which comprises coating at least one surface of an aluminum base sheet with an interlayer composition, anodizing the thus treated aluminum surface, optionally retreating the coated anodized aluminum plate with another stratum of the interlayer composition and finally applying to said surface a suitable light sensitive coating to yield the desired photosensitive lithographic printing plate.
It is well recognized that the substantial majority of photosensitive lithographic printing plates now produced are made from an aluminum based sheeting. This aluminum sheeting has, over the years, been found to suffer from certain disadvantages which prevent the direct application of the light sensitive coating to the aluminum base sheet. It has been found that upon direct application of the light sensitive composition to the aluminum base sheet, and subsequent exposure thereof to light, and the removal of the non-imaged areas for the preparation of a lithographic printing plate, the resultant printing plate has many undesirable characteristics which render it unacceptable for commercial use in the printing industry. Among the disadvantages suffered by such plates can be included the fact that the removed non-image areas do not possess sufficient hydrophilic and oleophobic characteristics, and therefore on printing will pick up background imperfections which will be imprinted on the final copy. In addition, since the aluminum is a rather soft metal, it does not have the resistance properties to sustain long press runs where more than about 100,000 copies are required. Further, the characteristic of the aluminum surface is such that a problem is encountered in achieving a strong bond between the light sensitive composition and the aluminum base sheet thus causing the image area of the plate to be dislodged from the surface, and renders the printed copy imperfect.
Heretofore, in the production of metal presensitized lithographic printing plates, it had been found beneficial to treat the surface of the metal substrate sheet, with a protective interlayer substance which imparts beneficial characteristics to the final lithographic printing plate thus produced. The prior art teaches that it is desirable to treat the metal sheet substrate surface receiving the light sensitive coating material, which when exposed to light and developed becomes the printing surface of the printing plate, with an undercoating substance that forms a strong bond with the metal sheet substrate and with the light sensitive coating material.
Many such undercoating treatments are known in the art for manufacturing longer running lithographic plates, and can be used on the sheets of this invention. U.S. Pat. Nos. 3,160,506, 3,136,636, 2,946,683, 2,922,715, disclose a variety of suitable materials for undercoating bonding substances onto plates and methods for applying them. Alkali metal silicate, silicic acid, alkali zirconium fluoride and hydrofluozirconic acid solutions presently are the most important commercial bonding substances. Those materials substantially improve the bonding of the light sensitive coating to the underlying metallic base which otherwise generally tends to have inadequate affinity for the coating. Of the various known bonding materials, the Group IV-B metal fluorides, the alkali metal salts and the acids thereof are preferred. In particular, the alkali zirconium fluorides, such as potassium zirconium hexafluoride, and hydrofluozirconic acid disclosed in U.S. Pat. Nos. 3,160,506 and No. 2,946,683 are used for preparing aluminum bases to receive a light sensitive coating.
One method in particular which is disclosed in the Jewett et al, U.S. Pat. No. 2,714,066 issued July 26, 1955, involves the use of a silicious overcoating on the aluminum sheet surface which acts as an interlayer bonding the photosensitive composition to the aluminum sheet surface. However, it has been found that this method does not completely solve the problem of bonding strength in that the bonding between the photosensitive composition and the aluminum base sheet has been found not to be strong enough to withstand long press runs, and in addition, tends to wear out or fail over an extended period of time.
The instant invention provides a novel method of manufacturing presensitized lithographic plates wherein a suitable interlayer composition such as have been hereinabove mentioned is coated directly to the surface of an aluminum sheet substrate, which may have previously been grained or etched. The coated substrate is subsequently electrolytically anodized, for example, in an aqueous sulfuric acid bath and then coated with a suitable lithographic photosensitive composition which is well known in the art.
According to the requirements of the present invention, the interlayer composition must be coated upon a substrate surface which is substantially free of any artificially produced oxide coating, for example, an anodic coating.
The resulting lithographic plate demonstrates a substantially extended shelf life, a protracted press life and improved clarity of the resultant printed image with improved oleophobic characteristics revealed in the non-image plate areas.
As another embodiment of the present invention, the coated, anodized aluminum substrate may again be coated with one of the above mentioned interlayer compositions prior to applying the photosensitive coating. This "sandwich" process exhibits even more profound beneficial characteristics of extended shelf life, press life, and clarity of image with improved oleophobic characteristics in the non-image plate areas.